There are two schools of thought in guitar back and side design and I will briefly discuss them. One group believes that the back and sides should be very rigid so the energy input of the top can passively reflect off of a very rigid and stiff surface.
The other school of thought is that the sides can be either flexible or rigid and then they are coupled to a flexible back to actively reflect the energy from the top.
Let’s put this in a different light. Let us pretend that the energy produced from the soundboard is a tennis ball bouncing around inside of your guitar. Now, let’s take the tennis ball and toss it at a concrete wall so that it bounces back to you. This would represent sound that is reflected from a rigid but passive back. It bounces back pretty easily, right? Now let’s take that same tennis ball and toss it at a trampoline so that it bounces off of it. Which surface do you think would bounce the tennis ball the furthest? The trampoline would, right? The trampoline would be an analogy of the guitar design using a light, flexible and active back.
The sides and back play differing roles in the acoustic guitar design. The sides, in my opinion, should be extremely stiff and rigid as opposed to flexible. They act to support the top and back of the guitar. The design is comparable to a drum where you have a very rigid rim assembly that supports a thin membrane on the top and bottom of the drum. The guitar sides do just that, support the top and back of the guitar PLUS they act as a structural member that supports the neck system.
The back plays a very active role in the design of the guitar. My theory is that the back wants to be thin and flexible so that it can couple with the top through the air chamber of the body. The back is 180* out of phase with the top to actively reflect the energy that the top produces. As the top moves inward the back moves outward and vice versa. To get an idea of how much of a role the back plays I would encourage you to try this experiment:
Lay your guitar across your lap with the top facing the ceiling. Now strum the open strings with a pick. Grab the headstock with one hand, while you continue to strum. Lift the neck upward so that the back is not touching your lap and observe the change in the sound and volume. As your guitar back is free to vibrate it will couple with the top and become active with the top. You can also repeat the same experiment by laying your guitar in its case and strum the open strings and then lift the neck up so the back is free. This experiment will quickly show you how important a role the back plays in guitar design.
Now that you have done this experiment, think about how this would apply to your guitar playing. How do you hold your guitar? Do you hold it close to your body so that the back is dampened? If so, the sound that you hear is mostly coming from the top of your guitar and your guitar back plays a very small passive role in the sound output of your guitar. The sound can be influenced and colored by the back wood based on the dampening properties of the wood. However, if the back is restricted from vibrating, I believe this will limit the volume and tone that your guitar is capable of producing.
If you play in such a way that the back of your guitar is held away from your body then you can take advantage of the guitar back’s active role in the coupling with the top. This will allow the total sound to couple with the back in such a way that it will “color” the tone of the top and add some rich lushness to the complete tonal package.
I used to think that classical guitarists looked so strange because they would hold their guitar on their left knee. It was only much later that I realized they did this to completely disconnect the guitar back from their body. These players are geniuses 😉 They understand the important and active role that the back plays in the output of the guitar. We, steel string players, can learn a thing or two from our nylon string friends!
Now that we have briefly discussed guitar tonewood side and back designs lets look at what we stock and offer our clients here at McKnight Guitars.
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